Thursday, March 22, 2012

DIY zinc plating of small parts

Many of the parts on my carburetors were in good shape, but the plating had worn off and they were rusty.  I think the original plating was cadmium.  You can buy easy-cad plating kits from places like Caswell and Eastwood, and that is certainly a good - though expensive - solution.

I decided that, with the internet as my teacher, I would try to plate small parts myself using a home-brew kit.

Zinc plating with vinegar is easy and pretty much non-toxic, so this is a good choice for the home mechanic.  Here is what I did.

First, I assembled:

1 Gallon jug of white vinegar ($1.50)

Coil of 3" wide 99.5% pure zince roofing metal strip (Home Depot, $30)

Bottle of Acid-Safe muriatic acid for cleaning parts (Lowes, $8?)

Can of acetone for degreasing parts ($5, Home Depot paint section)

Box of Epsom Salts for conductivity (grocery store pharmacy aisle, $4)

Box of saccharine packets as a brightener (Sweet and Low, grocery store, $4)

Box of baking soda ($2, grocery store)

Assorted 1/2 gallon plastic tubs

14g solid copper wire

1/4" copper tubing

12VDC bench power supply

Old 4VDC car cell phone charger

Coil about two feet of the zinc and make a fold so the end sticks up above the lip of the tub and folds over, with the coil resting on the bottom of the bucket and lining the walls.  This is your positive pole.  Drill two holes just below the rim of the bucket 180 degrees across from each other and insert a length of the copper tubing (like refrigerator ice maker tubing).  This is your negative pole.  Make some S-hooks from the wire that will be used to suspend the parts into the solution from the piece of copper tubing.

Fill the bucket with vinegar and add as much Epsom salts as it will take in solution.  Add 10 packets of Sweet and Low.

Get a small piece of scrap steel (plumbing fitting or something).  Clean the part up on a wire wheel and then dip it in the acetone to completely degrease it, and then set on a clean paper towel to dry - don't touch it again.  When dry, dip it into the muriatic acid and let it completely de-rust - be careful working around this muriatic acid.  I put some in a small plastic or glass container with a lid and try not to breathe when the lid is open.  Suspend the sacrificial piece of steel into the tub and connect the battery charger leads (get some small alligator clips to make this easier) - negative to the copper and positive to the zinc.  Turn the charger on.

The liquid around the part will start bubbling and the part will plate.  You are priming the liquid bath now - you need to get the ions from the zinc into suspension.  Leave this part going for several hours, then let the bath sit overnight.  When you do your parts later on, using the same method, they will plate faster.

Only plate one or two parts at a time.  The process is line of sight from the zinc to the part, and parts can mask each other if you add too many.  It will take from a couple of minutes to 1/2 hour to plate a part.  When done, dip the part in a solution of baking soda and water to neutralize the acid.  Then, "card" off the dull surface on a soft wire wheel and the part will be very shiny!  You can then clean and replate for a thicker longer lasting coating.

If you over-plate the part, it comes out lumpy and dull - looks more like galvanizing than a shiny chrome like surface.  You can plate-card-plate-card to build up a thicker, smoother coating.

I originally put a drop dish soap in the water as a "water wetter" but don't see that it makes much difference, and the soapy film on top of the water makes it hard to see what's happening, so I am now leaving it out.  Sugar or molasses is supposed to work as a brightener also, but I have read that the saccharine is better.  I have no idea how the chemistry for that works, so am doing it trial-and-error.  As I play around with it, I'll post the updates for any new lessons learned.

Pics to come....

Update No. 1 - I added some phosphoric acid ("prep and etch" - Home Depot) to the vinegar bath to lower the pH.  It increases the "throwing power" of the bath - you don't get as much of a chalky, lumpy plate - it's smoother and therefore shinier.  However, the acid also causes the bath to get murky and it's hard to see what's going on, and the part gets a little bit of black residue on it - but that cards off on the wire wheel OK.  I think you'd have to do several thin coats this way and card off the black deposits at intervals.  It might be a good idea to add the acid, but then reduce the voltage, which I will try.  The current is the thing you are really trying to control, and that is some value in mA for each square inch of plated surface.  I will have to set up a better station with a meter and variable power supply to really dial this in...but that is a project for another day later this summer, I think.


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  3. You are correct it is current that is likely the blame for the smut. Reduce it for these small parts and you should solve the problem.